Methine compounds have been conventionally used as a spectral sensitizing dye for silver halide photographic light-sensitive materials. Heretofore, with respect to the technology for improving the light absorptivity of silver halide grain, the following techniques are known. In order to improve the light absorptivity per one grain, the adsorption density of the sensitizing dye to a silver halide grain must be increased, however, a normal spectral sensitizing dye adsorbs to a monomolecular layer almost in the highest density filling state and does not adsorb any more.
In order to solve this problem, several proposals have heretofore been made. P. B. Gilman, Jr. et al., Photographic Science and Engineering, Vol. 20, No. 3, page 97 (1976), describes a technique of allowing a cationic dye to adsorb to the first layer and allowing an anionic dye to adsorb to the second layer using electrostatic force. U.S. Pat. No. 3,622,316 (G. B. Bird et al.) describes a technique of allowing a plurality of dyes to adsorb to a silver halide grain to form multiple layers and attaining the sensitization by virtue of Forster type excited energy transfer.
JP-A-63-138341 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-64-84244 (both Sugimoto et al.) describes a technique of attaining spectral sensitization using the energy transfer from a light-emitting dye.
These techniques have been created with an attempt to allow a dye to adsorb to a silver halide grain in excess of the saturated adsorption amount, however, the effect of elevating the sensitivity is not so high but on the contrary, the intrinsic desensitization disadvantageously increases.
On the other hand, U.S. Pat. Nos. 2,393,351, 2,425,772, 2,518,732, 2,521,944 and 2,592,196 and European Patent 565,083 described a 2 components-linked dye in which two or more non-conjugated dye chromophores are connected through a covalent bond. This technique is, however, not intended to improve the light absorptivity. For aggressively improving the light absorptivity, U.S. Pat. Nos. 3,622,317 and 3,976,493 (both G. B. Bird et al.) describe a technique of allowing a connection-type sensitizing dye molecule having a plurality of cyanine chromophores to adsorb to a grain and thereby increasing the light absorptivity, with an attempt to attain sensitization by the energy transfer, where, however, remarkable enhancement of the sensitivity is not obtained.
JP-A-64-91134 (Ukai et al.) proposes a technique of connecting a substantially non-adsorptive dye containing at least two sulfo or carboxy groups to at least one spectral sensitizing dye capable of adsorbing onto silver halide.
Also, in the spectral sensitization, JP-A-6-27578 (Vishwakarma) uses a 2 components-connected dye in which a cyanine dye adsorptive to silver halide and an oxonol not adsorptive to silver halide are connected, or European Patent 887700A1 (Parton et al.) uses a 2 components-connected dye in which an adsorptive cyanine dye and a nonadsorptive merocyanine dye or the like are connected using a specific linking group. In these techniques, however, the sensitivity is not sufficiently elevated by the energy transfer.
As such, sufficiently high elevation of the sensitivity is not yet achieved in any of these patents or publications and more technical development is being demanded.